Many computing systems having firmware, such as a basic input/output system (BIOS), perform power-on self test (POST) immediately after the computing system has been restarted or reset. The POST initializes and configures various components of the computing system, such as processors, memory, chipsets, and peripheral devices, and tests each such component before booting the operating system (OS) of the computing system. If there is a problem with a component, the BIOS may display an error message on a display device of the computing system.
The POST may not be able to proceed if there is an unexpected error within a component of the computing system, and can sometimes halt without providing any indication of the problem that has been encountered. The BIOS may further be unable to display an error message relating to a problem with a component on the display device if the problem is detected before the display device has been initialized. To determine the cause of such problems, some computing systems include a seven-segment light-emitting diode (LED) display that can indicate a relevant error via a POST checkpoint number.
The BIOS writes the checkpoint number to an input/output (I/O) port, such as I/O port 80h of a low-pin count (LPC) bus of the computing system. The single-byte data that is written to I/O port 80h is then displayed on the seven-segment LED display. When the computing system halts during the POST, the LED display indicates where the POST stopped executing. As such, the LED display is useful for debugging purposes.
However, many computing systems do not include such an LED display, making debugging more difficult. Furthermore, in some situations, the LED display even if present may not be useful. For example, some computing systems, especially server computing systems, include a secondary microprocessor, which is referred to as a service processor and/or as a baseboard management controller (BMC). The secondary microprocessor can detect when the computing system has stopped responding, and in response automatically reset the system. However, when the computing system is reset, the information displayed on the LED display is lost. As such, the computing system can be reset before a user has an opportunity to look at the LED display.